1. Field of the Invention
The present invention pertains to a medical ventilator with improved spontaneous triggering and cycling and to an improved method of triggering and cycling such a ventilator. In particular, the present invention pertains to a ventilator with one or more of the following features: (1) sensitivity to a patient initiated trigger that increases as the expiratory phase of the breathing cycle progresses, (2) adaptive adjustment of cycling criteria to optimize the cycling operation, and (3) triggering, cycling, or both based on patient effort, which is determined from cross-correlating multiple patient parameters.
2. Description of the Related Art
It is known to utilize a conventional ventilator or pressure support device to deliver a fluid, such as oxygen, air or other oxygen or breathing gas mixture, to an airway of patient to augment or substitute the patient's own ventilatory effort. It is further known to operate a conventional ventilator in a variety of modes to control the four basic operations of a ventilator, which are: 1) the trigger point, which is the transition from the expiratory to the inspiratory phase of the ventilatory cycle; 2) the inspiratory phase where the ventilator delivers the flow of breathing gas; 3) the cycle point, which is the transition from the inspiratory phase to the expiratory phase, and 4) the expiratory phase. There are four primary variables or parameters that are typically monitored and used to control how a ventilator performs one or more of these four operations. These variables are the volume, pressure, flow of fluid to or from the patient, and time.
In a typical life support situation, where there is substantially no spontaneous respiratory effort by the patient, a controlled mode of ventilation is provided, where the ventilator assumes full responsibility for ventilating the patient. In this mode of ventilation, the trigger and cycle point of the ventilator are determined based on time. In other situations, where the patient exhibits some degree of spontaneous respiratory effort, an assist mode or a support mode of ventilation is typically provided. Both of these modes of ventilation cause the ventilator to augment or assist in the patient's own respiratory efforts. In the assist mode, the determination of the ventilator trigger point is based on the action of the patient and the determination of the cycle point is determined based on time. In the support mode, both the trigger and the cycle points are patient based and not based on time. It is also known to use a combination of these two modes, referred to as an assist/control mode of ventilation. In this mode of ventilation, the ventilator triggers an inspiratory flow only if the patient fails to initiate a respiratory effort for a period of time. Thus, the trigger point is based on either a patient action or on time, if there is no patient action within a certain period of time.
In the assist, support, and assist/control modes of ventilation, it is important that the operation of the ventilator is synchronized with the patient's spontaneous respiratory effort, so that the ventilator triggers the inspiratory flow of breathing gas at or near the time the patient begins his or her inspiratory effort, and cycles to the expiratory phase of the breathing pattern at an appropriate time, preferably when the patient begins his or her expiratory phase of the breathing cycle. Conventional ventilators operating in an assist, support, or assist/control mode of ventilation typically monitor only one patient parameter, such as the pressure, flow, or volume, and use this single monitored parameter as a variable in determining when to spontaneously trigger the delivery of the inspiratory flow. Typically, the monitored parameter is compared to a threshold, and if the threshold is exceeded, the transition from expiration to inspiration (trigger) or from inspiration to expiration (cycle) is initiated. In other pressure support devices, the current value of the monitored parameter is compared to a previous value of the same parameter, so that the ventilator triggers or cycles based on the result of this comparison. U.S. Pat. No. 5,632,269 to Zdrojkowski et al. teaches this technique referred to as “shape triggering.”
This one-dimensional, i.e., one parameter, comparison of either pressure, flow, or volume to a trigger threshold is disadvantageous in that it is susceptible to random fluctuations in the monitored parameter, which may result in false triggers or cycles. In which case, an operator must intervene to reduce the trigger and/or cycle thresholds or ventilator sensitivity. However, reducing the ventilator's sensitivity can result in a greater amount of patient effort being needed before a spontaneous patient inspiration or expiration is detected, which is also disadvantageous, because a patient on a ventilator often has a weakened respiratory system to begin with.